The tropospheric jet response to prescribed zonal forcing in an idealized atmospheric model

We study the tropospheric jet response to the prescribed zonal forcing in an idealized dry model. The tropospheric jet shifts equatorward for the westerly (eastward) zonal torque on the equatorward flank of the jet in the troposphere. In contrast, the tropospheric jet moves poleward for the torque on the poleward flank of the jet in the troposphere, and for the torque outside tropics in the stratosphere. These jet movements project strongly onto the internal annular mode in the troposphere.

These jet movements can be explained by changes in the critical latitudes of the tropospheric eddies. For a westerly torque near the surface, the response can be thought of a zonally symmetric balance between the torque and surface friction at first, and the subsequent modifications on the eddies and the eddy-driven circulations. The zonally symmetric response in the zonal wind is nearly barotropic above the forcing. While the increased zonal winds in the subtropics allow the midlatitude eddies to propagate further into the tropics and result in the equatorward shift in the critical latitudes, the increased winds in the midlatitudes accelerate the eastward eddy phase speeds and lead to the poleward shift in the critical latitudes.

As for the torque in the stratosphere, the downward penetration of zonal winds to the troposphere displays a poleward slope for the subtropical torque, an equatorward slope for the high latitude torque, and less tilting for the midlatitude torques. These slopes are the signatures of anomalous wave propagation in the stratosphere. As the tropospheric eddies enter the stratosphere through the midlatitude tropopause, they transfer positive zonal wind anomalies from the torque region to the midlatitude lower stratosphere. These wind anomalies can modify the eddy phase speeds in the upper troposphere, and displace the tropospheric jet as seen for a tropospheric forcing.